An important limitation of current knowledge regarding the role of B cells in autoimmunity is that very little is known in human disease beyond the production of autoantibodies. Yet, accumulating data in diverse autoimmune diseases, including Systemic Lupus Erythematosus (SLE), indicates that B cells likely contribute to disease through multiple mechanisms that include both antibody-dependent and antibody-independent functions. The latter include antigenpresentation, T-cell activation and polarization, and dendritic cell modulation and we propose are critically mediated by the ability of B cells to produce cytokines. B cells may be deleterious through the production of pro-inflammatory cytokines and pathogenic autoantibodies or regulatory through the production of anti-inflammatory cytokines such as IL- 10 and TGFp and the expansion of Tregs and/or inhibition of effector T cells. Understanding the imbalance between these opposing B cell functions in disease is the overarching concept of this project and overall ACE proposal. It is founded on substantial preliminary evidence indicating that human B cells display phenotypic diversity that reflects division of labor for effector and regulatory functions, their sriking imbalance in SLE, and their reversal in patients experiencing good clinical response after B cell depletion and reconstitution. This imbalance could result from either: 1) numerical advantage in one population; or 2) a gain-of-pathogenic function and/or loss-of-regulatory function within a given population. Our preliminary view of a profile characteristic of active SLE includes the expansion of presumed effector subsets (switched CD27+ and CD27- cells) and/or the deficit of populations with regulatory potential (transitional, naTve and MZ cells). These assumptions will be addressed in Aim 1. We recognize however that surface phenotype is insufficient to determine the function of any given subset and therefore, Aim 2 will test the function of separate subsets in terms of cytokine producing potential and ability to modulate T cell function. Finally, we surmise that the fluctuating B cell imbalances that create a distinctive B cell signature in SLE PBL reflect recurrent systemic autoimmune responses dominated by antigen specificities known to fluctuate with disease activity. This contention represents the basis for Aim 3 which will test the contribution of specific autoreactivity to the expansion of effector and regulatory B cell subsets. Overall, we predict that effector B cells will be characterized by an activated phenotype and enhanced replication, potential for migration to non-lymphoid systemic tissues and production of pro-inflammatory cytokines. In contrast, regulatory B cells will display limited replication and specialize in the production of antiinflammatory cytokines. The information derived from these aims will be critical to understand whether a gain-ofpathogenic function and/or loss of protective B cell function help explain why clinical disease typically ensues years after the initial breakdown of tolerance, what type of B cell imbalance (either in number or function) underlies disease onset, exacerbation and remission and how to manipulate this knowledge for diagnostic, prognostic and therapeutic advantage.